feat: add authentication and book upload

- Session-based login (username/password, 30-day cookie)
- SQLite user store with bcrypt password hashing (modernc.org/sqlite)
- Per-user upload permission (can_upload flag)
- Admin CLI (cmd/admin) for user management:
  user add/list/delete/set-upload
- Upload handler for EPUB/PDF with path-traversal protection
- All routes protected by requireAuth middleware;
  /upload additionally requires requireUpload
- Login/logout UI, upload form, logout button in header
- New env var: USERS_DB (default: users.db, gitignored)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

go.sum

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